Document EP-A-0 770 514 describes an example of such a seat, in which the hinge mechanism can remain unlocked even after the control member has been released when titling the seat-back down forwards, e.g. to gain access to the back seats of the vehicle when the seat in question is a front seat. Thereafter, then when the seat-back is raised, the mechanism automatically locks itself again as soon as the first peg comes into register with the corresponding gap of the circular guide.
The seat described in that document gives complete satisfaction, however it nevertheless suffers from the drawback of always locking its seat-back in the same angular position whenever it is raised after being tilted down forwards, with this being independent of the prior adjustment of said seat-back.
The user of the seat must therefore readjust the inclination of the seat-back each time it is raised after being tilted down forwards.
Also, document EP-A-0 694 434 describes a seat provided with a hinge mechanism having toothed slugs, which mechanism enables the seat-back to be tilted down forwards and to automatically lock itself again in its previously-adjusted position.
The toothed slugs of the mechanism which enable the inclination of the seat-back to be adjusted engage in a set of teeth which is itself movable relative to the second cheek-plate and which can be secured to the second cheek-plate via other toothed slugs.
Under such circumstances, the two cheek-plates are therefore connected to each other via two stages of toothed slugs, which tends to reduce the strength of the mechanism, or to make it necessary to increase the weight and the cost of the mechanism in order to obtain high mechanical strength.